Handover is the process of transferring an ongoing radio communication session, being a voice call, a video call or any kind of user data exchange, from a serving cell, also referred to as the source cell, towards a new better-suited cell, also referred to as the target cell. For instance, as a mobile station is moving away from the coverage area of a serving cell, the radio signal from that serving cell weakens while the radio signal from another better-suited cell strengthens. When the difference in received signal powers between the two cells is past a predetermined threshold, a handover towards the better-suited cell is triggered.
For Long Term Evolution (LTE) mobile networks, an overview of the handover procedure and related message exchanges is described in §10.1.2 of the Technical Specification (TS) entitled “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, published by the 3rd Generation Partnership Project (3GPP) in June 2009, ref. 3GPP TS 36.300 V9.0.0.
In a first step, the source evolved-NodeB (eNB) configures the User Equipment (UE) measurement policy, e.g. the UE is configured to send MEASUREMENT REPORT on a regular basis and/or as soon as a handover event is detected. The measurement reporting period and/or the handover parameters for detecting the handover condition are transmitted by the serving eNB.
In a second step, the source eNB makes a decision to hand off the UE based on the MEASUREMENT REPORT message(s) received from the UE and/or on Radio Resource Management (RRM) criterion. The source eNB either issues a HANDOVER REQUEST message directly to the target eNB, or a HANDOVER REQUIRED message to the Mobile Management Entity (MME) which is relayed towards the target eNB as a HANDOVER REQUEST message, passing necessary information to prepare the handover at the target side, including the UE signaling context and the UE data service context.
In a third step, the target eNB configures the required radio resources, and optionally reserves a Random Access CHannel (RACH) preamble. The target eNB acknowledges the handover request by either sending back a HANDOVER REQUEST ACKNOWLEDGE message directly to the source eNB, or a HANDOVER REQUEST ACKNOWLEDGE message to the MME which is relayed towards the source eNB as a HANDOVER COMMAND message. The HANDOVER REQUEST ACKNOWLEDGE or the HANDOVER COMMAND message includes an Radio Resource Control (RRC) container, namely an RRC CONNECTION RECONFIGURATION message, to be transparently passed by the source eNB to the UE.
In a fourth step, the UE receives the RRC CONNECTION RECONFIGURATION message with necessary communication parameters to switch to the target cell. The UE performs synchronization to the target eNB and accesses the target cell via RACH, following a contention-free procedure if a dedicated RACH preamble was reserved, or following a contention-based procedure if no dedicated preamble was indicated.
In a fifth step, the target eNB responds with uplink allocation and timing advance value. When the UE has successfully accessed the target cell, the UE sends the RRC CONNECTION RECONFIGURATION COMPLETE message to the target eNB. The target eNB can now send/receive data to/from the UE.
In a sixth and last step, the target eNB informs the source eNB about the success of the handover procedure by sending a UE CONTEXT RELEASE message, which triggers the release of the radio resources by the source eNB.
In case of non-optimal handover parameter settings, high load in the surrounding cells or fast fading situations (e.g., tunnel road), handover signaling messages may be lost. For instance, the MEASUREMENT REPORT message sent by the UE upon detection of an handover condition may not reach the source eNB, meaning no target eNB is prepared to accept this UE. Sill for instance, the UE may not receive the HANDOVER COMMAND message sent by the source eNB, yet the target eNB is prepared to resume communication with this UE if required.
The Radio Link Failure (RLF) procedure as described in §10.1.6 of TS 36.300 allows a UE to resume communication with a prepared eNB, meaning an eNB which has admitted the UE during an earlier executed handover preparation phase.
If the receive signal is past an RLF threshold and radio connection gets lost with the serving eNB, then the UE tries during a first phase to resume first communication with the serving cell of the serving eNB. If so, the communication resumes as if no radio problem occurs. If the UE is unable to resume communication with the serving cell during the first phase, then the UE enters the RLF state and tries during a second phase to connect to the target cell or another cell which offers the best radio link quality, e.g. another cell of the serving eNB, by means of a RRC CONNECTION REESTABLISHMENT REQUEST message.
In the second phase, in order to resume activity and avoid going through RRC_IDLE, the following procedure applies:                The UE stays in RRC_CONNECTED.        The UE accesses the cell through the random access procedure;        The UE identifier used in the random access procedure for contention resolution, i.e. the Cell Radio Network Temporary Identifier (C-RNTI) of the UE in the serving cell where the RLF occurred+the Physical Cell Identity (PCI) of that serving cell+the short Message Authentication Code (short MAC-I) based on the encryption keys of that serving cell, is used by the selected eNB to authenticate the UE and check whether it has a context stored for that UE.        If the eNB finds a context that matches the identity of the UE, that is to say if the ENB has been prepared for that UE, the RRC connection can be resumed.        If no context is found, that is to say if the ENB was not prepared for that UE, the current RRC connection is released and the UE goes back to RRC_IDLE state (i.e., call is dropped).        